Micro manual
NEC PE calculations
The National Electrical Code (NEC) is the standard for practical safeguarding of persons and equipment. The areas that involve calculations are the components that are most frequently on the professional exams. Articles are used as broad categories from NEC 2005. The specific section & table numbers may change between versions of the NEC.
Conductor capacity: Article 212
1. Ampacity = (1.25 * largest Full Load Amps) + sum of all other current loads.
Overcurrent protection: Article 240
1. Overcurrent protection for specific equipment: Table 240.3
2. Protection of conductors shall be in accordance with ampacity from Table 310.16 ff
Grounding: Article 250
1. Sizing of grounding electrode conductor: Table 250.66
2. Sizing of equipment grounding conductor: Table 250.122
3. The maximum resistance of a grounding electrode according to Art 250.56: 25 Ω,
4. Conductor to be grounded - neutral or common: Art 250.26
Conductors for general wiring: Table 310.16 ff
1. Maximum capacity is shown in table at 30C
2. * AWG 14, 12, & 10 have maximum rating based on Art 240.4D, which is 5 A less than Table 310.16
3. Ampacity is rerated for different temperatures by the correction factors.
4. NM cable (Romex) is sized as 60C, regardless of insulation type
5. Alternate ampacity can be used under engineering supervision using Art 310.60D
Motors: Article 430
1. Motor full load current rating: Table 430.247 ff
2. Short circuit current = 6* full load current
3. Overload current = 1.5 * full load current
4. Maximum setting of motor branch circuit protection & ground fault protection: Table 430.52
5. Maximum locked-rotor current for selection of disconnecting means: Table 430.251
6. Locked rotor code letters for KVA per horsepower: Table 430.7
7. Environmental selection of motor controller enclosures: Table 430.91
8. Capacitor added to motor terminal: reduces total current from overload, so reduce size of overload only
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Tables: Chapter 9
1. Conduit dimensions and fill
2. Insulated conductors nominal dimensions for AWG, diameter, area
3. Conductor properties including AWG, area, Ohms/kft
4. Cable resistance and reactance in conduit
5. Power source limitations
Examples: Annex D
1. Single family dwelling with appliance, heating, air conditioning
2. Store building
3. Industrial feeders
4. Multifamily dwelling with demand factors on single-phase and three-phase
5. Generator field control
6. Mobile home
Marcus O. Durham, PhD, PE
Robert A. Durham, PhD, PE
09/25/05 ( ( (
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